From what I understand of the pages of speculation that didn't think big energy weapons from space, there are two possibilities.
One, a single restaurant caught fire and the flames remained contained but flaming debris and embers ignited the other two restaurants as well.
Two, an arsonist burned down all three buildings.
The trees survived likely due to that species having a high water content of the trees and low leaf flammability. Dead wood is more likely to catch fire than live wood. And from this distance, and perspective, we might not see that the trees around are, in fact, slightly scorched as a result of the fire.